http: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/http - from package: httpie

You may install the required package with command apt-get install {package}

Error in other languages:
http: bevel nie gevind nie
http: comandă negăsită
http: comando não encontrado
http: commande introuvable
http: command not found
http: käsku ei ole
http: không tìm thấy lệnh
http: komanda nerasta
http: Kommando nicht gefunden.
http: kommandot finns inte
http: komut yok
http: nie znaleziono polecenia
http: níor aimsíodh an t-ordú
http: no se encontró la orden
http: no s'ha trobat l'ordre
http: opdracht niet gevonden
http: parancs nem található
http: perintah tidak ditemukan
http: príkaz nenájdený
http: příkaz nenalezen
http: команда не найдена
http: командата не е открита
http: 命令找不到
http: コマンドが見つかりません